1. Field of the Invention
The invention relates to an optical device and a display apparatus, and more particularly to a projection lens and a projection apparatus.
2. Description of Related Art
In many types of display apparatuses, the projection apparatus has a feature of projecting an image with large size in a smaller appearance, and thus has the advantage of being irreplaceable. Since projection apparatuses use a projection lens to form an image from the light valve onto a screen, thus the image quality is deeply affected by the quality of the projection lenses. Therefore, a projection lens is a very important optical device in a projection apparatus.
Since a projection apparatus is usually set on a table or mounted on the ceiling, the light valve of the projection apparatus usually must have an offset greater than 100% relative to the optical axis of the projection lens. In other words, the light valve is located on a side of the optical axis, and the optical axis does not pass through the light valve. Therefore, according to the imaging principle, the image projected onto a screen will be located on the opposite side of the optical axis of the projection lens. This way, the image beam projected from the projection apparatus will not be projected onto the table or the ceiling, unable to form the image correctly.
An offset of 100% means when the light valve is precisely located on a side of the optical axis of the projection lens and the optical axis precisely intersects with the edge of the light valve. When the offset of the light valve is 100% or more, it means the offset of the light valve relative to the projection lens is greater, and so the size of the entire optical system is harder to reduce. In order to reduce the size of the optical system to make the projection system be applied in portable electronic devices, a portion of the projection lens on the side opposite to the offset direction of the light valve is cut off. However, this also results in cutting off the clear aperture of the image beam too much and reducing the brightness of the image greatly.
U.S. Pat. No. 7,580,206 discloses a fixed-focus lens including a first positive lens, a second negative lens, a third positive lens, a fourth positive lens, a fifth positive lens, and sixth negative lens. U.S. Pat. No. 6,439,726 discloses an optical system including a light source, a first optical portion, a second optical portion, and a digital micro-mirror device. U.S. Pat. No. 6,784,946 discloses an optical system including a light bulb, a first optical portion, a second optical portion, and a digital micro-mirror device.